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PROGRAMMING POWER
How to create a Web service to extract data from XML documents
By Steve Callan
Web services are a new breed of self-contained, self-describing, modular Web applications that can be published, located, and invoked across the Web. Web services perform specific functions, which can be anything from simple requests to complicated business processes. Once a Web service is deployed, other applications (and other Web services) can discover and invoke the deployed service. In other words, they are an ideal solution for providing a universal programmatic access to the Internet-a way for a program on one box to talk to any other program on any other box. Not only are they independent of the operating system, but also of the programs' internal implementations.
In concept, Web services and the Web share a lot of similarities. Web browsers allow people to access things on the Web using protocols like HTTP and HTML. With Web services, however, computers interact with one another directly, so a different set of protocols is needed.
XML, or Extensible Markup Language, is used to deliver the structured data to and from the disparate applications. It provides a universal data link that hides each program's internal code so that programs can communicate based on a protocol whose structure and rules they understand (behind the facade of a Web server, the XML message gets converted to a middleware request and the results converted back to XML for return to the requesting application).
A Web service makes a request for data using the Simple Object Access Protocol (SOAP), which is a lightweight XML based protocol for sending messages to and invoking methods on remote objects.
The third key protocol used in Web services is the Web Services Description Language (WSDL). Each Web services server maintains a WSDL file that acts as a directory to the available services on that server as well as of the rules for using each service. By retrieving the WSDL file through a SOAP query and understanding its contents through XML, the calling application is apprised of all the Web services available on that server and of how to request those services.
As an example, a company has a Web application that enables users to fill in and submit purchase orders using a browser. Using the Web services architecture, this system can be exposed as a Web service. A partner can then use the WSDL document to create an application that consumes this Web service (again using XML and SOAP as described by the WSDL document). This application would be integrated with its own order/inventory management system to automate the ordering process.
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